The present invention relates to the processing of semiconductor wafers. More particularly, the present invention relates to a germanium-doped boron phosphorus silicate glass layer formed over a semiconductor substrate and an improved method of forming the same. The present invention is particularly useful as a premetal dielectric layer but may also be applied to the formation of internal dielectric layers, passivation layers, and the like.
Silicon oxide is widely used as an insulating layer in the manufacture of semiconductor devices. A silicon oxide film can be deposited by thermal chemical vapor deposition (CVD) or plasma enhanced chemical vapor deposition (PECVD) processes from a reaction of silane (SiH.sub.4), tetraethoxysilane (Si(OC.sub.2 H.sub.5).sub.4), hereinafter referred to as "TEOS," or a similar silicon containing source with an oxygen containing source such as O.sub.2, ozone (O.sub.3), or the like.
One particular use for a silicon oxide film is as a separation layer between the polysilicon gate/interconnect layer and the first metal layer of MOS transistors. Such separation layers are referred to as premetal dielectric (PMD) layers because they are typically deposited before any of the metal layers in a multilevel metal structure. In addition to having a low dielectric constant, low stress and good adhesion properties, it is important for PMD layers to have good planarization characteristics.
When used as a PMD layer, the silicon oxide film is deposited over a lower level polysilicon gate/interconnect layer that usually contains raised or stepped surfaces. The initially deposited film generally conforms to the topography of the poly layer and is typically planarized or flattened before an overlying metal layer is deposited. A standard reflow process, in which the oxide film is heated to a temperature at which it flows, may be employed to planarize the film. Alternatively, a chemical mechanical polishing (CMP) or etching technique may be used.
Because of its low dielectric constant, low stress, good adhesion properties and relatively low reflow temperature, boron phosphorus silicate glass (BPSG) is one silicon oxide film that has found particular applicability in PMD layers. Standard BPSG films are formed by introducing a phosphorus containing source and a boron containing source into a processing chamber along with the silicon and oxygen containing sources normally required to form a silicon oxide layer. Examples of phosphorus containing sources include triethylphosphate (TEPO), triethylphosphite (TEP.sub.i), trimethylphosphate (TMOP), trimethylphosphite (TMP.sub.i), and similar compounds. Examples of boron containing sources include triethylborate (TEB), trimethylborate (TMB), and similar compounds.
Standard BPSG films have between a 3-5 weight percent (wt %) boron concentration and 3-6 wt % phosphorus concentration. At these concentration levels, the reflow temperature of known BPSG films is generally between about 800.degree.-900.degree. C.
Semiconductor integrated circuits currently being manufactured follow ultra high density (0.5 micron) design rules and circuits manufactured in the near future will follow even smaller design rules. At such small feature sizes, it becomes critical in some processes that reflow of PMD layers and other process steps be maintained at below 800.degree. C. to maintain shallow junctions and prevent the degradation of self-aligned titanium silicide contact structures.
BPSG films can be produced to flow at below 800.degree. C. by increasing the, concentration level of boron to 5% or above. At such increased boron concentration levels, however, the stability of the BPSG layer is adversely affected. Of course methods other than reflow, such as chemical mechanical polishing or etching, can also be used to planarize PMD oxide layers as appropriate.
From the above it can be seen that there is a need for a dielectric layer and method of forming such a layer, that has a low dielectric constant, high stability, low stress, good adhesion, and other characteristics sufficient for use as a PMD layer, that can be reflowed at a temperature of 800.degree. C. or below.
Thermal processes have been proposed in which germanium is added to a BPSG layer to lower the layer's reflow temperature. Such processes include low pressure chemical vapor deposition (LPCVD) and atmospheric chemical vapor deposition (APCVD) methods. In both processes, the use of TEP.sub.i as the phosphorus source results in the formation of unstable P.sub.2 O.sub.3 bonds in the silicon oxide film thereby producing a film of relatively low stability. A further problem with both these proposed processes is that deposition rate of Ge-BPSG films deposited using either processes (APCVD and LPCVD) is relatively low thus adversely effecting wafer throughput.